1. Field of the Invention
This invention relates to a distributed constant circuit, and more particularly to a distributed constant type multiple-line circuit, such as a multiplexer, formed as a dielectric block including a central conductor and filters placed around and connected to the central conductor.
2. Prior Art
Filters, multiplexers and so on, formed of variously shaped conductor layers on a dielectric block have conventionally been known and utilized. For example, see Japanese Patent Laid-open Official Gazette Nos. 4-82402 (1992), 5-29815 (1993), 6-6106 (1994) and 6-13802 (1994), and Laid-open Japanese Utility model Official Gazette Nos. 4-3804 (1992), 4-119104 (1992) and 6-29205 (1994). In particular, the Official Gazette No. 5-29815 (1993) discloses a circuit for a filter, multiplexer, power splitter or the like which is intended to prevent fluctuation in a resonant frequency caused by coupling with an external circuit. In order to prevent fluctuation in the frequency, the circuit employs, as a basic configuration, a dielectric resonator comprising a quarter-wavelength (.lambda./4) resonant line having one end shorted and the other end opened and a .lambda./4 coupling line having one end opened and the other end shorted wherein the coupling line is distributively inductively coupled in parallel with the .lambda./4 resonant line and the open end and the shorted end of the resonant line are paired with the shorted end and the open end of the coupling line, respectively. When the circuit is formed as a multiplexer or a power splitter for instance, it is configured in such a manner that an input line is positioned along the center axis of a dielectric block having a thickness equal to .lambda./4, with conductors deposited on peripheral surfaces and a rear surface thereof, resonant lines each having a length equal to one quarter of the wavelength of a resonant frequency are arranged on the front and rear sides of the input line, and output lines are closely arranged on the front and rear sides of the resonant lines.